How do you get into a junior community college?

Typically (in the US system, anyway), you must have a high school degree or GED. You can apply to college before you have actually received the degree as long as you expect to have the degree before the college courses would start. Typically in that situation, your admission would be conditional on proof of high school degree completion. Once you have a high school degree (or expect to receive one soon), you can apply to college. There are a multitude of resources available on the web, at community centers, libraries or from high school guidance councilors to help you choose where to apply. Once you've picked where you want to apply, you get the application (many schools' applications are online, others still use paper), fill it out, send it in (usually with an application fee, high school transcript, and three letters of recommendation), and wait to hear back.

To get into more competitive colleges, you must have an exemplary high school record (grades, extracurriculars, etc). However, there are many less competitive institutions which accept nearly any student holding a high school diploma or GED.